A wide range of services for telecommunication subscribers is made possible by the Integrated Services Digital Network (ISDN). With ISDN, terminal equipment can be connected to the network by two 64 Kbps channels (B-channels) and one 16 Kbps channel (D-channel). The B-channel information is primarily circuit-switched voice or data. The information in the D-channel is accumulated in the network to be packet switched. The D-channel information can be packet-switched data or it can be control information, which is used by the network to control the switching of the B- and D-channels. Accordingly, the D-channel can be engaged in multiple concurrent communications to control multiple D-channel packet communications and the B-channel communications. In order to associate D-channel packets with the correct communication controlling program, each packet includes a Terminal End Point Identifier (TEI), a Service Access Point Identifier (SAPI), and a call reference value. The call reference value is selected by the originator of the communication and uniquely identifies the communication at that originator. The SAPI defines whether the associated packet includes signaling, data, or administrative information. The TEI identifies a particular user terminal using the D-channel in which the packet is conveyed. One TEI, called the broadcast TEI, is never assigned to a specific user terminal. When a request is received by an ISDN switch from an external source for a communication to a user terminal, a TEI is selected by the switch to be included in the first packet sent to the user terminal. In accordance with ISDN standards, the TEI selected is the broadcast TEI. When a user terminal receives a packet including the broadcast TEI, it formulates and transmits a D-channel packet including its own (not the broadcast) TEI so that future references can uniquely identify the call.
When the response packet from the user terminal is received by the switch, it must be associated with the external request to which it relates. This is an exceedingly difficult task, since the TEI of the response packet does not match the broadcast TEI of the first packet transmitted from the switches. Tables can be created which associate user TEIs and D-channels with those receiving packets having broadcast TEIs. But the search of such tables when the number of active calls is large requires valuable system time. This can become additionally complicated when multiple TEIs are being used on the same D-channel. Accordingly, an efficient method of properly associating response, packets having different TEIs than the message which stimulated the response with the appropriate control structure for the packet is needed.